This invention relates to a prosthetic femoral component of the type which is applied without a stem in the medullary canal, which is considered to be conservative and bone sparing.
For present purposes the definition of a conservative femoral hip prosthesis is a prosthesis which leaves sufficient bone in place for it to be eventually replaced by a more conventional femoral hip prosthesis with a medullary stem normally intended for primary (non-revision) application. A bone-sparing femoral hip prosthesis is one which limits the removal of viable bone by conserving some of the femoral head, removing only sufficient bone to resect the diseased tissue and to effect a satisfactory anchorage.
The use of a femoral hip prosthesis which functions without a stem in the medullary canal date from the first total hip prosthesis reported by Wyles in 1937. This hip prosthesis was fitted following a high resection of the femoral head and was stabilized with a straight stem which passed along the femoral neck and out below the greater trochanter, where it was attached to a bone plate secured on the lateral cortex of the femur. The Wyles hip restored the femoral head with a bearing diameter deliberately smaller than the natural femoral head it was replacing. Only six cases were ever performed using this device since the clinical outcome was not impressive.
Another femoral hip prosthesis design following that of Wyles was the Judet prosthesis, developed in France and used in the period 1945-55. A high neck resection was used with this prosthesis, which attempted to restore the femoral head to its natural diameter for use as a hemiarthroplasty. The prosthesis comprised an acrylic (low modulus) head and a short straight stem which passed along the femoral neck. The prosthetic head included a trough around the stem attachment to the head, which was used to seat and locate the prosthesis on the prepared proximal end of the femoral neck. Early breakage caused the stem to be given a stainless steel core support. Later failures saw the device breaking out through the inferior femoral neck. All versions of this prosthesis suffered from premature wear of the acrylic head.
High neck resections, i.e. those conserving the femoral neck, were also used by femoral hip prostheses with stems passing into the medullary canal, notably the designs of Pipino (1978) and Freeman (1985). These hip prostheses were implanted both cement-free and with cement, but did not attempt to restore the femoral head to its natural diameter, being used as total hip replacements with a head of smaller dimensions. Since these femoral hip replacements do place a stem in the medullary canal, they are not considered to be conservative, although the stem on the Pipino design was very short.
Designs of the femoral hip prostheses which have attempted to secure the replacement of the femoral head without a stem in the medullary canal follow the design of Vincent and Munting reported in 1982, which is still in clinical use. With this design, a portion of the femoral neck is preserved and shaped with a notch to provide seating for the implant. The prosthesis is used as a total hip replacement and replaces part of the femoral neck and the femoral head with a head of smaller diameter than the natural head. The prosthesis is used uncemented and is fixed with a large screw through the lateral cortex into the body of the prosthesis. The prosthesis is intended to sit on the remaining cortex of the neck and is stabilized by fins parallel to the axis of the neck which pass into the remaining diaphyseal cancellous bone. The bone engaging surfaces are provided with a hydroxyapatite coating to promote bone ongrowth to augment fixation.
The Vincent-Munting prosthesis is considered to be conservative but not bone sparing, according to the definitions given above. The only type of femoral hip prosthesis which has been developed which is conservative and bone sparing is the femoral cap used in prostheses such as the ICLH (Freeman, 1973), the THARIES (Amstutz, 1976), the Wagner (Wagner, 1973), the Zephyr (Aubriot, 1977) and the Gerard (Gerard, 1975). This type of prosthesis comprised a metal cap with a part-spherical external form and different internal forms and was used both cemented and uncemented. The bearing surface of the femoral cap was always near to anatomical size, therefore the cap could be used as a hemiarthroplasty. Mechanical loosening through stress concentration at the bone interface were reported as well as resorption of epiphyseal bone beneath the cap. The cause of the bone resorption was associated with disruption of the blood supply to regions of bone as a result of the surgical technique. Often the cap was used to articulate with a polyethylene liner in the acetabulum, and with this an additional failure mode of osteolysis at the bone interface with the prosthesis was caused by the ingress of polyethylene debris.
A development of the femoral cap design was the inclusion of a short stem to the cap. Examples of such designs include the TARA hip (1970""s) and, more recently the McMinn hip (1990""s).
An alternative design approach for the femoral cup is presented in U.S. Pat. Nos. 4,532,660 and 4,662,888, which describe a stemless femoral hip prosthesis intended to load the bone naturally. The first design required the resection of most the femoral head and part of the neck, the later design required only the resection of the proximal portion of the femoral head up to the epiphyseal scar plate. In the later design, a low modulus material between the bone and the femoral cap was used to transfer load with a more physiological force distribution onto the trabecular structure of the proximal femur. In practice, too little bone was removed for adequate surgical exposure of the acetabulum without excessive soft tissue damage. Furthermore, controlled exposure of the three-dimensional epiphyseal scar plate proved to be too complex and the design was never developed into an implant.
Cemented intramedullary fixation of femoral hip prostheses has now approximately 30 years successful clinical results and is the benchmark against which new designs of hip implants are assessed. Early problems of implant fracture, corrosion, cement mantle integrity and excessive bearing wear have now been largely resolved and the main problem which limits the life expectancy of conventional femoral hip prostheses is aseptic loosening. Nevertheless, since premature failure of the reconstruction may occur due to loosening, eventual revision of the prosthesis, particularly when used for younger patients (under 65), must be considered.
The revision of cemented stemmed femoral hip prostheses is challenging, particularly as a result of needing to remove all the cement. In fact, cementless stems with intramedullary fixation have been developed to simplify the revision procedure. Such devices require increased surgical precision compared with cemented hip prostheses and have their own failure modes such as pain, loosening and subsidence.
It is the likelihood of subsequent revision for the younger and more active patient which makes a conservative, and indeed bone sparing, femoral hip prosthesis an attractive option. In theory, such a device may be revised with a conventional primary stemmed hip prosthesis without the need for bone grafting or other augmentation. Indeed, there is no reason why conservative hip designs could not be at least as safe and efficacious as intramedullary stemmed hip designs. However, attempts so far to develop a conservative, bone sparing femoral hip prosthesis have encountered significantly worse results due to premature loosening of the femoral component (and acetabular component).
The present design seeks to provide a conservative, bone sparing femoral hip prosthesis that addresses the problems encountered by previous designs. The prosthesis includes an insert portion which is designed to control the ,transfer of load to the femur so as to avoid stress concentration at the bone interface. The insert portion is sized so that it replaces all the epiphyseal bone thereby minimizing the risk of bone resorption due to disrupted blood supply. It is also tapered so as to self seal under load so as to restrict the ingress of debris leading to osteolysis.
In addition to addressing the deficiencies of previous designs, the present design seeks to simplify the surgical technique so as to achieve better reproductability of results to minimize the trauma (e.g. loss of blood, post-operative infection) associated with the procedure.
Hip replacement is usually performed with a large exposure. Early post-operative infection is no longer a significant problem, but the time to heal such a major wound is significant. Some surgeons now implant conventional stemmed devices with as small an incision as they possibly can. After the femoral head and neck have been removed, only narrow tools are needed to prepare the femoral canal and there is easy access to the acetabulum. However, the bone sparing femoral hip prosthesis designs generally necessitate reverting to a wider exposure for two reasons. Firstly, the preparation of the outside of the femoral head involves bulkier instruments. Secondly, the femoral head obstructs access to the acetabulum. More cutting of soft tissues attaching the femur to the pelvis is needed to maneuver the femoral head out of the way.
The present invention is intended to provide a femoral hip prosthesis which can be employed in a method of fitting which includes cutting away the natural femoral head to expose the circular cross-section of the neck at the base of or at a mid point of the head. This allows much improved access to the acetabulum, thereby reducing the length of the required incision and minimizing the soft tissue dissection necessary to allow the remaining femoral head to be levered out of the way. The shape of the insert portion of the prosthesis is designed so as to allow it to be fitted to the bone accurately following a simple, non-bulky, reproducible reaming operation. As such, the close fit will resist micromotion and act in support of the self-sealing taper design to impede the ingress of debris. The fact that non-bulky instruments may be used allows a less-invasive surgical technique to be employed.
According to the present invention a prosthetic femoral component for location in a prepared socket in a femur which has been resected at a position on the proximal side of its neck includes a tapered insert portion and a proximal head portion, the proximal end of the insert portion being adapted for location in the prepared socket and having a maximum dimension in a plane normal to the distal-proximal axis of the neck which is larger than the minimum dimension of the neck in a parallel plane.
Thus, the component according to the present invention takes advantage of the bone at the periphery of the socket which enables the insert to be accurately and firmly located in the bone. The presence of the bone at the outer edges of the socket helps to stabilize the component. Preferably the tapered insert portion is flared outwardly in the proximal direction. The tapered insert portion can be dimensioned to pass through the neck of the femur with which it is to be used or it can stop short of it depending upon the requirements.
In one preferred embodiment the tapered insert portion has a smooth finish. This can enable it to sink into the bone if it is inserted with the use of cement. A void centering arrangement can be provided in the manner shown in U.S. Pat. No. 5,092,892.
In any case, the proximal end of the head portion can be of generally spherical shape and have a bearing surface for co-operation with an acetabular socket. The proximal end of the head portion can be provided with a male taper to receive a matching female taper on the part-spherical bearing element.
The bearing element can have a spigot adapted to engage in a bore provided in the head portion of the spigot and bore can be tapered to provide an engaging fit. In one preferred embodiment the spigot is elongated and extends through the head portion and into the tapered insert portion.
Alternatively the proximal end of the head portion can be substantially hemispherical as is the inner wall 26 of the bearing element which has a tapered elongate spigot adapted to engage in a tapered bore in a head portion to provide an engaging fit and the spigot can extend through the head portion and into the tapered insert portion to provide stability.
In any of the preceding constructions any of the parts can be made of metal, of a synthetic plastics material or a ceramic material.
In the alternative construction the prosthetic femoral component can be formed as a single component.
Preferably the tapered insert portion has a general axis which is inclined to the central axis of the head portion in a plane radial thereto.
The tapered insert portion can be non-circular and be adapted to prevent rotation relative to the bone.
Thus, the cross-section of the tapered insert portion can be elongate in a plane extending normal to the central axis. With this type of construction the cross-section of the tapered insert portion can be substantially rectangular, oval or figure-of-eight shaped or any other desired cross-section.
The tapered insert portion can be arranged to extend radially away from the distal rim of a head portion towards the central axis.
In another preferred arrangement the distal side of the head portion is formed as a trough which extend around part of the tapered insert portion and the distal portion of the insert portion can be formed with a concave taper.
If desired the proximal end of the tapered insert portion can be provided with a series of radially outward extending steps or fins.